I. Field of the Invention
The invention relates generally to a method of and apparatus for reducing frequency errors associated with an inter-system scan. The invention is useful for reducing frequency error when performing an intersystem scan between a continuous frequency division duplex system, for example a WCDMA system, and a differently duplexed system, for example a time division duplex system such as GSM, but is not limited to such an application.
II. Description of the Related Art
In the following, reference will be made to the WCDMA and GSM standards, but it will be appreciated by those possessed of the relevant skills that the invention is not limited to those standards and may be applied to inter-system scans between other different communication systems. The terms “mobile unit” and “mobile phone” are also used and, of course, cover mobile phones, but also cover other devices capable of communicating with a cellular system in the transfer of voice or information data.
Most if not all equipment used in communications systems includes an internal frequency reference circuit that is adjustable to allow the equipment to adjust the timing of its operations to align with that of other equipment within the communications system. For example in a cellular telephone system the mobile units include a reference frequency circuit, which is commonly an adjustable oscillator whose output frequency can be trimmed within a specified range using an control voltage input, to enable the timing within the mobile unit to be aligned with that of the serving cell or network. The frequency reference circuit is commonly a phase-locked loop based around a voltage controlled temperature compensated crystal oscillator (VCTCXO), an accurate oscillator that allows the range to be specified in terms of parts per million (ppm).
The mobile unit will, of course, be in communication with the system when it is active during a call. The system needs to “know” where the mobile unit is when it is not receiving or making a call so that incoming messages can be sent to it. Mobile units therefore continue to operate in an idle mode when a call is not being made so that data can -be paged to it from time to time. The mobile unit has to maintain time alignment with the WCDMA system, in order to correctly monitor for paging data from the system for the mobile unit. The VCTCXO and phase-locked loop are usually very accurate and need only be subjected to very minor corrections, essentially remaining at a constant frequency while the phone remains in active or idle communication with the system.
During certain situations, for example when leaving the coverage area of an initial system and entering the coverage area of a new system, it becomes necessary to monitor the signals of the surrounding cells belonging to the new system so that a controlled handover can take place between the two systems. While it would be possible to do this from within the system, such an approach would place an undue burden on the system because it would have to monitor both active and idle mobile units. The mobile units themselves therefore do this monitoring.
There are many different standards available for use in cellular communications systems including the widely established GSM standard and the newer CDMA standards including WCDMA. System users expect to have continuous service even when they roam outside the area of coverage of the service company to which they subscribe. In order to meet this expectation, so called multi-mode phones have been developed that are capable of communication in, say, both a WCDMA system and a GSM system.
The continuous nature of WCDMA signals means that there is no natural break in the signals that would allow scanning for signals from other systems. Furthermore, there is no common synchronization between a WCDMA system and a GSM system. Different systems are asynchronous in that there is no predefined relationship between the timing and phase of the signals of the systems.
One way of overcoming this would be to apply a post-reception phase correction to the data before decoding is done. This could be done by using a hardware rotator to correct for phase offset as the received data is sampled, i.e. receive a sample, rotate it and then store it in memory for decoding. A problem with this approach is that a hardware rotator requires additional hardware, and hence uses extra silicon area, and therefore adds additional cost and complexity to the circuit (which is usually provided in the form of an ASIC).
Another way would be to rotate the received samples in memory before the decoding takes place, i.e. bring in all of the samples and then apply a rotation algorithm. A problem with this approach is that a signal processing rotation algorithm such as this requires code to be written, memory in which to store it and a DSP (digital signal processor) powerful enough to run it in the required time. This approach therefore consumes DSP memory and energy from the battery.
One proposal for the WCDMA standard to overcome this problem is to use a method known as compressed mode to create “free-space” periods during which the receiver can be re-configured to scan for other systems. Thus, during a free space period, the receiver is tuned to a GSM channel and a measurement of characteristics of the channel is performed. At the end of a “free-space” period, the receiver must be configured back to the WCDMA channel of the original system in order to ensure uninterrupted service. In order to do this, the receiver needs to be phase locked before reception is attempted Therefore, the phase-locked loop (or other reference oscillator) must be re-tuned and re-synchronized both when changing the phone from WCDMA to GSM and when changing the phone from GSM back to WCDMA.
When the phone is retuned to scan for other serving systems, a number of actions have to occur:                1. Initialize receiver and re-program PLL to new (GSM) frequencies.        2. Scan for data to give coarse frequency synchronization.        3. Scan for data to give fine frequency correction.        4. Decode GSM system access parameters.        5. Re-configure receiver and re-program PLL to original (WCDMA) frequencies.        6. Re-acquire and re-synchronize to the WCDMA system.        
This retuning results in a frequency error being introduced into the synchronization of the phone to the WCDMA system, making the re-acquisition more difficult when returning to the WCDMA channel. One proposal for reducing this problem when switching between GSM and WCDMA modes is to extend the free space period as much as possible for the GSM scan, to minimize the response time of circuits, and to rely on the receiver to re-synchronize using an frequency or time tracking auto frequency control (AFC) algorithm.
However, this approach is not entirely satisfactory at least for the following reason. When the scanning for data occurs in the above actions 2 and 3, the phase-locked loop is fine-tuned using a control voltage input. This fine-tuning results in a frequency error being introduced into the synchronization when the phone is reconfigured to return to the WCDMA system in the above action 5. Consequently, the re-acquisition in action 6 is more difficult to perform.